HappyNumberSolver.java

public class HappyNumberSolver {
    public static void main(String[] args) {
        int count = 0;
        int sum = 0;

        for (int i = 1; i <= 9999; i++) {
            if (isHappy(i)) {
                count++;
                sum += i;
            }
        }

        long result = (long) count * sum;
        System.out.println(result);
    }

    // 행복 수 판별 함수
    private static boolean isHappy(int n) {
        java.util.Set<Integer> seen = new java.util.HashSet<>();
        while (n != 1 && !seen.contains(n)) {
            seen.add(n);
            n = sumOfSquares(n);
        }
        return n == 1;
    }

    private static int sumOfSquares(int n) {
        int sum = 0;
        while (n > 0) {
            int digit = n % 10;
            sum += digit * digit;
            n /= 10;
        }
        return sum;
    }
}